“Rust and Bone” tries to tell two stories at once, and in doing so, it neglects the more interesting one.

As the film opens, we meet Ali (Matthias Schoenaerts) and his son Sam (Armand Verdure). Ali is neglectful and irresponsible, a bonafide loser who’s as unfit for fatherhood as any man on earth. He and Sam live on the street, scrounging for food, but soon make it to the home of Ali’s sister (Corinne Masiero) in the south of France, where they settle down.

While working as a bouncer at a nightclub, Ali meets Stephanie (Marion Cotillard), a pretty killer whale trainer who loves the attention of men. After a fight at the club, he drives her home and leaves his phone number, but nothing comes of it. Then Stephanie loses her legs in a freak accident (nothing graphic is shown, thankfully), and a few months later she calls Ali out of the blue. He’s now training in mixed martial arts; they strike up a relationship that deepens over time, even if he has the emotional sensitivity of a rat.

Part of the reason the movie is so uneven is that it comes from two different sources. In 2005, author Craig Davidson published a collection of short stories, “Rust and Bone.” The title story followed a once-promising boxer who breaks all 27 bones in his hand and is propelled into a series of underground bare-knuckle fights. Another story, “Rocket Ride,” tells of a male killer whale trainer whose leg is torn off by an orca.

Director and co-writer Jacques Audiard found enough symbolic similarity in these stories to make small changes and meld them and, in the sense of two people with broken bodies helping each other find meaning in their lives, he might have been on to something. Too bad that isn’t the movie he made.

What he gives us instead is essentially Ali’s story with Stephanie in a unique supporting role. We see poor and desperate Ali womanize, make more bad decisions, work, fight, and so on. And this is a mistake. Not only is Cotillard a much better actor than Schoenaerts, but her Stephanie is a much more interesting character. She goes from craving the attention of others and loving her job to not wanting to be seen and learning to live with a disability. The transition is huge and far more interesting than sad-sack loser Ali trying to make ends meet, and the immensely talented Cotillard is more than game enough to pull off the performance. If she were featured more prominently, the story would have balance and be more enjoyable. Better yet, cut Ali out altogether and just focus on Stephanie.

Aside from Audiard’s directorial failure, there’s Cotillard’s fantastic performance, which by itself makes “Rust and Bone” almost worth seeing. Unfortunately, there’s not enough of her to warrant even that qualified recommendation. Take this as an example of a director not giving us enough of the best he has to offer — and his movie suffering mightily.